The most popular waste water treatment methods of the prior art are classified into the following two types: a type in which the coagulating agent is first added to the waste water and then the produced flocs are precipitated; and a type in which air is introduced into water after adding a coagulating agent to let the produced flocs float up.
According to the former type of method, the precipitated dense sludge is removed out of the system by using a thickener, while in the latter type of method the floated sludge is taken out of the treating plant by scumming, using a flotator or the like. However, both thickener and flotator devices have deficiencies, e.g. they are costly to install, operation time is extensive, sludge produced has too much water content, the turbidity of the purified water is a little higher, large floor space is required, and such devices are also high in operating cost. Further, since the concentrated sludge contains large quantities of water (e.g. 98-99%), the sludge needs to be dehydrated by using a filter press, Oliver filter or other like means, and the resultant treated water still has turbidity on the order of 10 ppm. Therefore, such treated water must be subjected to further filtration for obtaining high purity water.